Adhesives are used for a wide variety of applications in the graphic arts industry such as for bookbinding, and for puzzle and gameboard laminations.
One specific application where adhesives are used is for casemaking. The type of adhesive most often used for this application is referred to as "animal glue," although water based emulsions are also used. Animal glue is an amorphous organic colloid derived from animal protein and extended with various plasticizers including water in amounts of between 40% and 50% by weight. Animal glue is usually applied using a roller which is supplied adhesive from a reservoir heated to between about 60.degree. C. and about 85.degree. C. This roller then transfers it to one or two other rollers which will ultimately come into contact with the covering material- Water based emulsions would be applied in a similar fashion, except the reservoirs would not require heat. "Animal glues" and waterbased emulsions both contain substantial amounts of water. Moisture from the adhesive can migrate into the binding board from the adhesive resulting in warpage of the cover. This migration tendency increases as the amount of water present during the process increases. This has an obviously negative impact on the resultant book.
U.S. Pat. No. 4,944,994 to Flanagan issued Jul. 31, 1990 teaches the use of radial styrene-butadiene block copolymer based hot melt pressure sensitive adhesives, and U.S. Pat. No. 4,722,650 to Allen et al. issued Feb. 2, 1988 teaches the use of A-B-A block or A-B-A-B-A-B multiblock styrene-butadiene copolymer based hot melt pressure sensitive adhesives for use as casemaking adhesives for "hard" bound books. Major problems can occur with adhesives of this type when used in the casemaking application. First, these pressure sensitive hot melts contain oil which can cause staining and "strike-through" of the cover material especially if the cover material has a natural finish which means the material is of a more porous nature. Staining refers to the oil leaching from the hot melt whereas strike-through refers to the adhesive itself penetrating through the material.
The equipment currently used in this industry for casemaking is designed for using "animal glues." The equipment is capable of application temperatures of no greater than about 130.degree. C. Although the hot melt adhesives of the type taught in Flanagan have melting temperatures of about 90.degree. C., the viscosities are too high at this temperature to be used effectively with the application equipment and require temperatures of greater than about 150.degree. C. for effective application. This would therefore require equipment modification which could result in higher cost and is therefore disadvantageous to the book binder.
The second problem occurs during what is called "hot stamping" or embossing of the cover. Hot melt adhesives of this nature have melting points of about 90.degree. C. which are well below "hot stamping" temperatures of typically about 175.degree. C. The temperatures used for hot stamping may be as low as about 120.degree. C., but this is unusual. This can cause the adhesive to vacate the area where the stamping occurs due to the high temperatures used for this process, resulting in voids and causes the cover material to ultimately separate from the binder board.
A third problem occurs due to the lack of heat resistance of these hot melt pressure sensitive adhesives. If a book is exposed to ambient temperatures in the summer, in a car for example, where temperatures can exceed 75.degree. C. the turn-ins may begin to delaminate.
Puzzles and gameboards can be laminated on equipment similar to that used for casemaking and would therefore have similar requirements.
Water based emulsions are also currently used for passport thread-securing and for casing-in. Use of these adhesives for casing-in can have negative consequences for the book due to the substantial amount of water present. Too much moisture leads to saturation of the paper which further results in cockeling or waviness within the first or last twenty pages due to expansion and contraction of the paper as it dries. Moisture travels inward and causes warping of the paper within the book. Migration of moisture into the cover or case can have a similar effect on the cover wherein positive or negative warping occurs which just refers to the direction of the warp either in toward the pages of the book, or outward.
European Pat. Application 0,279,279 published Aug. 24, 1988 teaches the use of styrene-butadiene-styrene block or multiblock copolymer based adhesives containing a tackifying resin, oil, and optionally a wax for use in casing-in operations, and European Pat. Application 0,358,907 published Mar. 21, 1990 teaches the use of a radial styrene-butadiene block copolymer based adhesive containing a tackifying resin and oil for use in casing-in operations. These formulae are hot melt pressure sensitive adhesives and the disadvantage of using such formulae for casing-in is that the oil content leads to staining of the endsheets. Adhesion will also decrease over time as the oil, which makes a major contribution to the level of tack, wicks out of the adhesive, leaving less oil to plasticize the block copolymer and tackifier.
Further disadvantages can result with the handling of hot melt pressure sensitive adhesives like those formulae above. Due to the fact that a pressure sensitive adhesive remains tacky at ambient temperatures, excess adhesive squeezed out beyond the endsheet on to the case during compression will result in the first page of the book adhering to the case of the book, resulting in undesirable fiber tear from the page. Pressure sensitive adhesives also adhere to equipment, clothes, and skin and are difficult to remove.
Other applications where adhesives are used include "lining-up," "tightbacking," and bonding sides or joints. U.S. Pat. No. 4,660,858 to Flanagan et al. issued Apr. 28, 1987 teaches the use of A-B-A block or A-B-A-B-A-B multiblock styrene-butadiene copolymer based hot melt pressure sensitive adhesives and EP Pat. App. No. 0,355,468 A1 teaches the use of radial styrene-butadiene block copolymer based hot melt pressure sensitive adhesives for lining-up applications. U.S. Pat. No. 4,712,808 to Bek-Forrest et al. teaches the use of A-B-A block or A-B-A-B-A-B multiblock styrene-butadiene copolymer based hot melt pressure sensitive adhesives for hinge joint applications. The aforementioned disadvantages of using hot melt pressure sensitive adhesives applies here as well. These inventions do not teach how to make and use a reactive hot melt polyurethane adhesive for lining-up, tightbacking, and bonding sides and joints.
Another application in which water based emulsions are currently used is for passport thread-securing, but these adhesives set very slowly and require drying.
Perfect binding, which is also referred to in the trade as one shot bookbinding, is the most common method by which books are bound. This involves stacking the sheets or signatures of the book, holding them in a clamp to form a book block, applying adhesive to the backbone of the book block, and then adhering a cover to the backbone of the book block before the adhesive is set.
Hot melts are commonly used for perfect binding because the fast rate of set required for this method can be achieved with hot melt adhesives. "Animal glues" and water based adhesives, in contrast, set slowly and are not as commonly used for this application. Although hot melt adhesives based on thermoplastic polymers, such as ethylene vinyl acetate or block copolymers are most commonly used, it has also been taught that reactive hot melt polyurethane adhesives can also be used.
U.S. Pat. No. 4,942,195 to Flanagan et al. issued Jul. 17, 1990 teaches the use of a radial styrene-butadiene block copolymer based hot melt adhesive for use in the binding of books. Binding in this instance appears to encompass one shot or two shot bookbinding. Hot melts are commonly used because of the fast rate of set that can be achieved. Reactive hot melt polyurethane adhesives, in contrast, are not ideally suited for binding because of their slow cure rates.
Articles have been published on the use of hot melt polyurethanes for perfect binding. Two such articles are Bindery Meets Customer Demands with Polyurethane-Reactive Adhesive, Adhesives & Sealants Industry and General Bindery Shifts to PU Reactive Hot Melts, Adhesives Age, August, 1994. Although the benefits of using reactive hot melt PU adhesives is expounded upon in these articles, the disadvantages are not discussed. Reactive PU hot melts have slow cure rates. Prior to cure the books cannot be stressed. If movement or shifting should occur, the books would no longer be satisfactory for shipment which often occurs before the adhesive is cured. Quality control tests cannot, therefore, in some circumstances be performed prior to shipment, and there is no guarantee that the books will meet specifications. Rounding is a process wherein a curvature is made on the spine of the book block. This occurs after addition of crash and kraft to reinforce the book block, and prior to casing-in of the book wherein a cover is bonded to the book block. It is important that the adhesives remain flexible enough to allow the curvature to be maintained for the prolonged life of the book. These adhesives also allow rounding of the book block for an extended period after adhesive application. European Pat. application 0 361 122 teaches the use of low viscosity reactive polyurethane hot melt adhesives for use in the rounding application.
The above examples illustrate that attempts have been made to replace hot melts with reactive hot melt polyurethanes. However, no attempts or suggestions have been made to replace water based emulsions or animal glues with reactive hot melt polyurethanes.
Although it is known in the art how to make and use reactive hot melt polyurethane adhesives for binding and rounding, it is not known in the art how to make and use reactive hot melt polyurethane adhesives for casemaking, casing-in, gluing-off or thread-securing, lining-up, tight-backing, bonding sides and joints, and laminating puzzles and gameboards because of the disadvantages noted above.
The present inventors have now surprisingly found that reactive hot melt polyurethanes can successfully replace water based emulsions and animal glues. Reactive hot melt polyurethanes cure slowly and are more ideally suited for those applications where slower setting products such as water based emulsions and animal glues are used. Hot melts, in contrast, set more quickly and are generally used for faster applications such as binding and rounding of books, and replacing them with reactive hot melt polyurethanes has not been very commercially successful. One problem which may occur with these polyurethane adhesives is that the bonds do not set quickly enough and are prone to "creep" which simply means that the bond is not set enough prior to cure and the bond line may shift. It is the object of the present invention to teach how to utilize reactive hot melt polyurethane adhesives for these applications. It is a further object of the present invention to teach hot melt moisture curing polyurethane compositions which may be applied at low temperatures and which exhibit a further benefit of forming bonds which are resistant to "creep."